This invention concerns exhaust gas recycling systems, referred to as EGR systems, for diesel engines and relates in particular to such a system in which the quantity of fuel injected by its injection system pump is controlled by a pneumatic governor.
Reference is made to the conventional EGR system illustrated schematically in FIG. 1, wherein atmospheric air taken in through air cleaner 1 flows through venturi passage 2 and intake pipe 3 and enters combustion chamber 5 of cylinder 4 through inlet valve 6. The air cleaner 1, venturi passage 2, and intake pipe 3 can be regarded as an air infeed. Combustion gases escape into exhaust pipe 7 through an exhaust valve, not shown, and are discharged to the atmosphere through a muffler 8.
In this prior system, part of the exhaust gases is recycled to the combustion chamber through branch line 9, orifice 10, EGR valve 11 and intake pipe 3, the amount of recycled gases being dependent on the extent with which EGR valve 11 is open where all other conditions are held constant. EGR valve 11 opens under the influence of the negative pressure produced by vacuum pump 13 and supplied under control by amplifier 12. Injection pump 14 for injecting fuel into combustion chamber 5 through the injection nozzle is communicated to air cleaner 1 through line 15 and also to venturi passage 2 through line 16. Pump 14 includes a vacuum governor which controls the pump according to the difference between the two pressures sampled on lines 15 and 16, one being more negative than the other, to control the rate and quantity of fuel injection in the known manner: the governor actuates and positions the fuel control rack in the pump.
The EGR characteristics of such a conventional system are illustrated in the graphs of FIGS. 2, 3, 4 and 5, all being based on the data taken on a particular diesel engine.
In the graph of FIG. 2, the differential pressure mentioned above (i.e., the difference between pressures on lines 15 and 16) is plotted as P.sub.1 on the vertical axis and the injection quantity is designated as Q on the horizontal axis. In the conventional EGR system, the characteristic shown in FIG. 2 is determined primarily by the operating characteristic of the vacuum governor fitted to the injection pump. In other words, the vacuum governor is present to actuate and position the control rack in response to the said differential pressure according to the indicated characteristic.
The system shown in FIG. 1 uses a vacuum amplifier 12 and a vacuum pump 13 because the negative venturi pressure is not high enough to actuate directly the exhaust gas recycling valve 11 in the conventional system. It has been heretofore customary in diesel engines with conventional exhaust gas recycling of this type to boost the negative pressure available from the venturi passage and this need has been met by such an amplifier and a vacuum pump. Referring to the system of FIG. 1, this boosting or amplification is accomplished by admitting two negative-pressure inputs to amplifier 12: one is air cleaner pressure, varying with engine speed as shown in FIG. 13, from line 15 through branch line 15a; and the other is venturi pressure from line 16 through branch line 16a. Operating with these inputs, vacuum amplifier 12 controls the negative-pressure output applied to EGR valve 11.
The graph of FIG. 3 shows the output P.sub.2 of the vacuum amplifier on the vertical axis and fuel injection quantity Q on the horizontal axis to indicate the relationship between the amplifier output and the injection quantity. It must be pointed out that this relationship or characteristic is that which is preset, and is similar to that shown in FIG. 2. With this amplifier output characteristic, if recycling is to be effected in the P.sub.2 range above the level where P.sub.2 is equal to negative 200 mm Hg, the flowrate E of recycled gases will vary with P.sub.2 in a manner depicted by the curve of FIG. 4, in which the vertical axis is scaled for flowrate E (liters/minute) and the horizontal axis for output P.sub.2 (negative mm Hg).
In the theoretical system characterized by FIGS. 2, 3 and 4, the proportion of recycled gases to the total intake of the cylinder, i.e., the EGR ratio, will vary with engine speed N according to the curves of FIG. 5, there being five curves representing 0%, 25%, 50%, 75% and 100% of the rated engine load. Note that the variation of percent EGR ratio differs for different levels of engine load. The percent EGR ratio, designated R, is defined by this expression: ##EQU1##
From the curves of FIG. 5, it can be seen that the EGR ratio increases with decreasing speed in the low speed range and also with increasing speed in the high speed range. This relationship between the EGR ratio R and the engine speed N is believed to arise from the fact that, for a given engine load and a given negative-pressure input to the EGR valve with a consequently constant valve lift, the amount of recycled gases increases with speed in the high speed range because the pressure in the exhaust pipe increases. Under the same conditions, in the low speed range the total intake volume of the engine becomes smaller relative to the amount of recycled gases as engine speed decreases. This relationship or system characteristic has heretofore presented two problems, which are:
(1) If the system is set with a specific EGR ratio calculated to cover the entire range of engine speed, the actual ratio will increase in the low speed range because of the characteristic described above. This will deteriorate fuel combustion, resulting in an increasingly large proportion of hydrocarbons in the exhaust gases, giving these gases a black color and an offensive odor.
(2) In the high speed range, the larger the engine load, the higher the pressure inside the exhaust pipe; and, where the EGR valve is of a type opening at a certain level of actuating vacuum, the higher the engine speed, the higher will be the load level at which this valve opens. In other words, the engine will tend to lack power as its speed rises in the high-speed maximum-load range.